Climate change is slow, and relies on a few highly indirect mechanisms like agricultural failures and famines to kill civilization-level-significant amounts of people. This is a hard ceiling for it.
An advanced AI threat is a civilization of one. It can act, adapt and adjust rapidly, like humans do. Potentially faster than humans do. It has access to all the tools that human civilization has already invented that it can reproduce, obtain or subvert - and potentially more. It's not limited to having just one big slow lethality mechanism. It can unleash multiple civilization-scale threats in a quick succession, and keep piling them until recovery becomes impossible. Humans can adapt to a lot, but an intelligent adversary adapts back.
The threat class isn't the same, and it's not even close. Climate change is an "otherwise avoidable death and suffering" risk. AI is an actual "extinction of humankind" risk.